A device that measures a phase or a phase difference of a periodic input signal with a digital circuit (hereinafter referred to as a “phase measurement device”) is a most fundamental component in wireless communication, high-frequency signal processing, and high-accuracy frequency measurement. Since a frequency can be calculated by differentiating phase information obtained by the phase measurement device by time, it is also used as a frequency counter. In wireless communication or wired communication, high-speed high-capacity communication has been sought in recent years, and a phase noise of a reference signal source is one of factors limiting performance. Therefore, for example, a device (hereinafter referred to as a “phase noise measurement device”) that measures a phase time history, power spectrum, Allan deviation, and the like of one periodic input signal is used in order to evaluate the reference signal source.
Further, a phase measurement device that measures a phase difference between two periodic input signals (hereinafter also referred to as a “phase difference measurement device”) is widely used as an element constituting many measurement/control instruments. For example, in a laser heterodyne displacement measurement device, the phase difference measurement device is applied to demodulate displacement of a measurement target from an optical modulation signal. Even in a power control system, it is necessary to measure AC power, voltage and current signals which periodically vary, and the phase difference measurement device is incorporated in a part of the system. Particularly, with the demand for miniaturization of the power control system, it is desired to have a simple structure while maintaining the accuracy of the phase difference measurement device.
A phase-locked loop (hereinafter referred to as a “PLL circuit”) is a circuit which is widely used for a communication instrument or a measurement device, and a phase difference measurement device that measures a phase difference is applied as an internal element thereof. In some physical quantity measuring sensors such as an angular velocity detecting sensor (also referred to as a gyro sensor), since the PLL circuit is incorporated therein, there are sensors including the phase difference measurement device therein. In addition, a phase difference measurement device that measures a phase difference between digital pulse signals (which indicate signals whose waveform is a square wave) is widely used mainly for communication instruments or the like.
In the phase measurement device or the phase difference measurement device, since performance such as measurement accuracy, resolution, a dynamic range, or the like influences final system performance, research and development for increasing convenience while increasing the performance of the measurement devices have been conducted. In recent years, a device that converts an input signal to digital data through an AD (analog to digital) converter and then measures a phase or a phase difference by digital processing (hereinafter referred to as a digital phase measurement device or a digital phase difference measurement device) has started to appear, particularly, due to convenience of implementation and interface with a computer.
As a technique of measuring a phase or a phase difference of a periodic input signal by processing digital data, several methods have been known from the past, and it can be roughly classified into a demodulation technique, a counting technique, and a zero crossing technique.
First, the demodulation technique will be described. In the demodulation technique, a reference signal is generated in a circuit, and a phase of an input signal is detected by multiplying the reference signal by the input signal, and it is possible to implement a phase measurement device by performing the process on a periodic input signal. In this method, generally, high accuracy phase measurement is possible, but there is a problem in that demodulation is unable to be performed when a frequency of the input signal is significantly different than the reference signal. Further, when an amplitude of the input signal changes in a short time due to influence of noise or the like from the outside, there is a problem in that the measurement accuracy is adversely affected. In addition to the method of multiplying the reference signal as described above, for example, there is a method of generating a quadrature phase signal by Hilbert transformation and calculating a phase by an inverse tangent arithmetic operation, but these methods can be classified into the demodulation technique and have a similar problem.
Next, the counting technique is an already known technique of counting the number of zero crossings of a periodic signal using a counter and calculating a phase from a count value. It can be implemented with a very simple circuit configuration on the basis of the principle, but since it is possible to calculate only a phase of an integral multiple of a signal period, the accuracy is limited. In this regard, various researches have been conducted to improve the accuracy of the counting technique.
For example, a technique of arranging a PLL circuit at a preceding stage and applying the counting technique after multiplying the frequency of the periodic input signal has been proposed, and it is possible to amplify a minute phase change of the input signal and improving the accuracy through this technique. However, since a response speed of the PLL circuit is limited, reliability of measurement decreases when the frequency of the periodic input signal varies drastically (when the phase variation is severe). Further, a method of improving accuracy by a combination of a simple counting technique and a counting technique using a higher frequency clock has been also proposed, but there is a problem in that the high frequency clock is necessary, and a circuit configuration and signal processing are complicated. Furthermore, a technique of calculating and correcting a decimal part by a linear interpolation operation before and after a zero crossing point in order to correct a count value for which only an integer value is obtained has been also proposed. However, even though the correction is added, since only a count value at an end point within a certain measurement time (also referred to as a gate time) is used, a measurement resolution is limited.
On the other hand, in the zero crossing technique, a time of a point at which the periodic input signal crosses zero (zero crossing) is measured, and the phase of the signal is calculated on the basis of it. Specifically, the phase difference of the periodic input signal is calculated on the basis of data accumulated in a memory using the fact that a time interval in which the signal crosses zero is proportional to a reciprocal of a frequency. However, in this method, since the phase is estimated from data between two neighboring measurement points among the data accumulated in the memory, there is a problem in that the measurement time is limited by a memory capacity. Further, since it is necessary to perform conversion from a time of zero crossing to a phase, a computation load is large due to signal processing or the like, and it is difficult to implement a real-time process.
In order to improve the performance of the phase measurement device, it is effective to integrate a plurality of techniques so that the disadvantages of the respective techniques are complemented. In particular, integration of the counting technique capable of coping with the high-speed phase change and the zero crossing technique capable of coping with the high accuracy phase measurement is extremely effective since it is possible to implement a measurement device that can achieve both a high-speed process and accuracy without using a complicated digital process as in the demodulation technique.
From this viewpoint, for example, a technique of detecting a phase difference through a combination of a counting technique using an up/down counter as a phase difference measurement device applied to a laser heterodyne interferometer and a zero crossing technique using a triangle wave generated from an input signal is disclosed in Patent Document 1.
Further, a technique in which a digital phase difference measuring unit is used as a part of a phase-locked loop, AD conversion is performed on an input signal, and a phase difference between a sinusoidal input signal and a clock held therein is obtained by processing it with a clock generating unit, a phase comparing unit, and a phase correcting unit is disclosed in Patent Document 2. In a principle of the phase difference measurement unit, the input signal is first digitized by the AD converter, and an input signal digital value is generated. Then, a “sign clock” indicating whether a digital value of the input signal is positive or negative is generated by the clock generation unit. Then, the phase comparing unit performs counting on the basis of a high-speed “count clock” held therein using a sign clock. At the same time, the phase correcting unit calculates a phase correction value through a linear interpolation operation for data before and after a zero crossing point of the input signal digital value, calculates a sum of an output value of the phase comparing unit and the phase correction value, and obtains a desired phase difference.
Patent Document 3 discloses a method of performing AD conversion on an input signal and performs detection and correction of a phase error as a device that detects a phase error (synonymous with a phase difference in this specification). The detection of the phase error is carried out by an equalization unit having a predetermined equalization characteristic, a binarization unit that binarizes a signal output from the equalization unit, and an arithmetic operation unit that calculates a desired phase error signal by a metric operation from an output of the equalization unit and the binarization unit. As a method of realizing the correction, there is a method of determining whether or not a previous phase error history is within a predetermined range and performing correction so that an error falls within the range when an error outside the range is detected.
As a frequency measurement method and device, a method of performing an interpolation operation on amplitude values before and after the zero crossing point of the periodic input signal, calculating a time of the zero crossing point gradually, and calculating a frequency of the periodic input signal from a reciprocal of a difference in the time of the zero crossing point is disclosed in Patent Document 4. This method is also included in the “zero crossing technique”.
Patent Document 1: Japanese Patent No. 2946675
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2012-217121
Patent Document 3: Japanese Patent No. 5468372
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2007-232380